1. Field of the Invention
The present invention relates to apparatus and methods for sensing rotation of a wheel, which is in particular a wheel of a vehicle.
2. Description of the Related Art
In modern vehicles, a plurality of vehicle subsystems is employed, which are for example activated depending on a vehicle movement state. The vehicle subsystems may for example be tire pressure sensors, which are disposed in a tire and supplied by a battery. Preferably, such systems are therefore not activated before the vehicle is set in motion in order to lower energy consumption and thus realize more inexpensive electronic systems.
In tire pressure control systems, tire pressure sensors are built in the tires of the vehicle in order to measure the tire pressures of the respective wheels preferably during vehicle movement and then send them for example to a central evaluating unit. To this end, a sensor with a radio frequency transmitter is built in an interior space of the wheel, for example. In the vehicle there is a receiver evaluating the radio frequency data sent out from the wheel and informing the driver about the current tire pressure. The sensor (a module) in the wheel is mostly supplied from a battery intended to last for example ten years. The module therefore is only allowed to send when this is necessary, i.e. when the vehicle is in motion. For this reason additional sensors are employed in the wheel module, which sense vehicle movement.
For sensing movement of the vehicle, centrifugal force sensors may be employed. Here, a centrifugal force is exerted on the module by rotation of the wheel. By evaluation of the centrifugal force, it may now be ascertained whether the wheel is moving, the movement is permanent, or the wheel is in the rest state. The centrifugal force sensors may either be mechanical or electrical. In a mechanical solution, for example a spring is compressed by a small weight, whereby for example an electric contact closes. Disadvantageous in this approach, however, is short life, since the spring return forces become lower and lower with increasing usage. It is also disadvantageous in this approach that the electric contacts are closed mechanically. With this, there may be contact problems when for example the contacts are oxidized or when for example contact transition resistance becomes too large.
With the electrical solution, various acceleration sensors are employed, such as piezoelectric sensors, in which due to mechanical deformation an electric signal is provided, which indicates the wheel movement. Disadvantageous in this concept, however, are the high costs caused both by the piezoelectric devices and by the complex evaluating electronics.
For activating the tire pressure sensors action systems, so-called wakeup systems, may also be employed. Here, activation signals are sent from a transmitter disposed in a wheel housing to a receiver built in the wheel module. If the receiver in the wheel module gets a request signal, which has been sent out from the vehicle, the wheel module sends the pressure data from the wheel to the vehicle. Disadvantageous in this approach, however, is that a complicated receiver has to be built in the wheel module. On the one hand, thereby system costs are increased. On the other hand system operation costs thereby increase, because the receiver in the wheel module constantly requires energy to be able to receive the activation signal at any time. This reduces the life of the module. Furthermore, a battery with large capacity is needed to provide the required amount of energy. Moreover, a transmitter, which is built in the vehicle to wake up the module wheel, is costly and expensive to integrate in the vehicle systems.
For activating the tire pressure sensor, pressure versus time (waveform) in the wheel may also be analyzed. When driving away for example the temperature and the pressure in the wheel tire change. This change may be evaluated by logic integrated in the sensor, for example, which activates the transmitter. It is disadvantageous here, however, that the sensor constantly measures the pressure and the temperature. Hereby increased current consumption arises, which has to be covered by a larger and thus more expensive battery. Moreover, the technology required for this is complex and parameter-dependent, so that a stable system performance is not guaranteed in every vehicle situation. This is for example the case when, depending on roadway surface and driving performance, the pressure differences are great and quickly variable, so that the evaluating logic does not achieve a stable state, which may lead to the system not reacting, for example. This leads to the further disadvantage that such systems are not safe.